The present invention relates to liquefied gas storage tanks and in one aspect relates to tanks especially adapted for storing liquefied gases at cryogenic temperatures at near atmospheric pressures (e.g., liquefied natural gas (xe2x80x9cLNGxe2x80x9d)).
Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Liquefied natural gas (LNG) is typically stored at cryogenic temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.) and at substantially atmospheric pressure. As used herein, the term xe2x80x9ccryogenic temperaturexe2x80x9d includes any temperature of about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.) and lower. Typically, LNG is stored in double walled tanks or containers. The inner tank provides the primary containment for LNG while the outer tank holds insulation in place and protects the inner tank and the insulation from adverse effects of the environment. Sometimes, the outer tank is also designed to provide a secondary containment of LNG in case the inner tank fails. Typical sizes of tanks at LNG import or export terminals range from about 80,000 to about 160,000 meters3 (0.5 to 1.0 million barrels) although tanks as large as 200,000 meters3 (1.2 million barrels) have been built or are under construction.
For large volume storage of LNG, two distinct types of tank construction are widely used. The first of these is a flat-bottomed, cylindrical, self-standing tank that typically uses a 9% nickel steel for the inner tank and carbon steel, 9% nickel steel, or reinforced/prestressed concrete for the outer tank. The second type is a membrane tank wherein a thin (e.g. 1.2 mm thick) metallic membrane is installed within a cylindrical concrete structure which, in turn, is built either below or above grade on land. A layer of insulation is typically interposed between the metallic membrane, e.g., of stainless steel or of a product with the tradename Invar, and the load bearing concrete cylindrical walls and flat floor.
While structurally efficient, circular cylindrical tanks in their state-of-practice designs are difficult and time consuming to build. Self-standing 9% nickel steel tanks, in their popular design where the outer secondary container is capable of holding both the liquid and the gas vapor, albeit at near atmospheric pressure, take as long as thirty six months to build. Typically, membrane tanks take just as long or longer to build. On many projects, this causes undesirable escalation of construction costs and length of construction schedule.
Recently, radical changes have been proposed in the construction of LNG terminals, especially import terminals. One such proposal involves the building of the terminal a short distance offshore where LNG will be off-loaded from a transport vessel, and stored for retrieval and regasification for sale or use as needed. One such proposed terminal has LNG storage tanks and regasification equipment installed on what is popularly known as a Gravity Base Structure (GBS), a substantially rectangular-shaped, barge-like structure similar to certain concrete structures now installed on the seafloor and being used as platforms for producing petroleum in the Gulf of Mexico.
Unfortunately, neither cylindrical tanks nor membrane tanks are considered as being particularly attractive for use in storing LNG on GBS terminals. Cylindrical tanks typically do not store enough LNG to economically justify the amount of room such tanks occupy on a GBS and are difficult and expensive to construct on a GBS. Further the size of such tanks must typically be limited (e.g. to no larger than about 50,000 meters3 (approximately 300,000 barrels)) so that the GBS structures can be fabricated economically with readily available fabrication facilities. This necessitates a multiplicity of storage units to satisfy particular storage requirements, which is typically not desirable from cost and other operational considerations.
A membrane-type tank system can be built inside a GBS to provide a relatively large storage volume. However, a membrane-type tank requires a sequential construction schedule wherein the outer concrete structure has to be completely built before the insulation and the membrane can be installed within a cavity within the outer structure. This normally requires a long construction period, which tends to add substantially to project costs.
Accordingly, a tank system is needed for both onshore conventional terminals and for offshore storage of LNG, which tank system alleviates the above-discussed disadvantages of self-standing cylindrical tanks and membrane-type tanks.
In published designs of rectangulartanks (see, e.g., Farrell et. al., U.S. Pat. Nos. 2,982,441 and 3,062,402, and Abe, et al., U.S. Pat. No. 5,375,547), the plates constituting the tank walls that contain the fluids are also the major source of strength and stability of the tank against all applied loads including static and, when used on land in a conventional LNG import or export terminal or a GBS terminal, earthquake induced dynamic loads. For such tanks, large plate thickness may be required even when the contained liquid volume is relatively small, e.g., 5,000 meters3 (30,000 barrels). For example, Farrell et al. U.S. Pat. No. 2,982,441 provides an example of a much smaller tank, i.e., 45,000 ft3 (1275 meters3), which has a wall thickness of about xc2xd inch (see column 5, lines 41-45). Tie rods may be provided to connect opposite walls of the tank for the purpose of reducing wall deflections and/or tie rods may be used to reinforce the corners at adjacent walls. Alternatively, bulkheads and diaphragms may be provided in the tank interior to provide additional strength. When tie rods and/or bulkheads are used, such tanks up to moderate sizes, e.g., 10,000 to 20,000 meters3 (60,000 to 120,000 barrels), may be useful in certain applications. For traditional use of rectangular tanks, the size limitation of these tanks is not a particularly severe restriction. For example, both Farrell, et al., and Abe, et al., tanks were invented for use in transport of liquefied gases by sea going vessels. Ships and other floating vessels used in transporting liquefied gases typically are limited to holding tanks of sizes up to about 20,000 meters3.
Large tanks in the range of 100,000 to 200,000 meters3 (approximately 600,000 to 1.2 million barrels), built in accordance with the teachings of Farrell et al. and Abe, et al. would require massive interior bulkheads and diaphragms and would be very costly to build. Typically, any tank of the type taught by Farrell et al., and Abe, et al., i.e., in which the tank strength and stability is provided by the liquid containing tank exterior walls or a combination of the tank interior diaphragms and liquid containing tank exterior walls, is going to be quite expensive, and most often too expensive to be deemed economically attractive. There are many sources of gas and other fluids in the world that might be economically developed and delivered to consumers if an economical storage tank were made available.
Bulkheads and diaphragms in the interior of a tank built in accordance with the teachings of Farrell, et al. and Abe, et al., would also subdivide the tank interior into multiple small cells. When used on ships or similar floating bodies, small liquid storage cells are of advantage because they do not permit development of large magnitudes of dynamic forces due to ocean wave induced dynamic motion of the ship. Dynamic motions and forces due to earthquakes in tanks built on land or on sea bottom are, however, different in nature and large tank structures that are not subdivided into a multitude of cells typically fare better when subjected to such motions and forces.
Accordingly, there is a need for a storage tank for LNG and other fluids that satisfies the primary functions of storing fluids and of providing strength and stability against loads caused by the fluids and by the environment, including earthquakes, while built of relatively thin metal plates and in a relatively short construction schedule. Such a tank will preferably be capable of storing 100,000 meters3 (approximately 600,000 barrels) and larger volumes of fluids and will be much more fabrication friendly than current tank designs.
The present invention provides substantially rectangular-shaped tanks for storing fluids, such as liquefied gas, which tanks are especially adapted for use on land or in combination with bottom-supported offshore structures such as gravity based structures (GBS). Also methods of constructing such tanks are provided. A fluid storage tank according to this invention comprises (I) an internal, substantially rectangular-shaped truss frame structure, said internal truss frame structure comprising: (i) a first plurality of truss structures positioned transversely and longitudinally-spaced from each other in a first plurality of parallel vertical planes along the length direction of said internal truss frame structure; and (ii) a second plurality of truss structures positioned longitudinally and transversely-spaced from each other in a second plurality of parallel vertical planes along the width direction of said internal truss frame structure; said first plurality of truss structures and said second plurality of truss structures interconnected at their points of intersection and each of said first and second plurality of truss structures comprising: (a) a plurality of both vertical, elongated supports and horizontal, elongated supports, connected at their respective ends to form a gridwork of structural members, and (b) a plurality of additional support members secured within and between said connected vertical and horizontal, elongated supports to thereby form each said truss structure; (II) a grillage of stiffeners and stringers arranged in a substantially orthogonal pattern, interconnected and attached to the external extremities of the internal truss frame structure such that when attached to vertical sides of the truss periphery, the stiffeners and stringers are in substantially the vertical and horizontal directions respectively, or in substantially the horizontal and vertical directions respectively, and (III) a plate cover attached to the periphery of said grillage of stiffeners and stringers; all such that said tank is capable of storing fluids at substantially atmospheric pressure and said plate cover is adapted to contain said fluids and to transfer local loads induced on said plate cover by contact with said contained fluids to said grillage of stiffeners and stringers, which in turn is adapted to transfer said local loads to the internal truss frame structure. As used herein, a plate or plate cover is meant to include (i) one substantially smooth and substantially flat body of substantially uniform thickness or (ii) two or more substantially smooth and substantially flat bodies joined together by any suitable joining method, such as by welding, each said substantially smooth and substantially flat body being of substantially uniform thickness. The plate cover, the grillage of stiffeners and stringers, and the internal truss frame structure can be constructed from any suitable material that is suitably ductile and has acceptable fracture characteristics at cryogenic temperatures (e.g., a metallic plate such as 9% nickel steel, aluminum, aluminum alloys, etc.), as may be determined by one skilled in the art.
A tank according to this invention is a substantially rectangularshaped structure that can be erected on land and/or fitted into a space within a steel or concrete GBS and that is capable of storing large volumes (e.g. 100,000 meters3 and larger) of LNG at cryogenic temperatures and near atmospheric pressures. Because of the open nature of trusswork in the tank interior, such a tank containing LNG is expected to perform in a superior manner in areas where seismic activity (e.g. earthquakes) is encountered and where such activity may induce liquid sloshing and associated dynamic loads within the tank.
Advantages of the structural arrangement of the present invention are clear. The plate cover is designed for fluid containment and for bearing local pressure loads, e.g., caused by the fluid. The plate cover transmits the local pressure loads to the structural grillage of stringers and stiffeners, which in turns transfers the loads to the internal truss frame structure. The internal truss frame structure ultimately bears all the loads and disposes them off to the tank foundation; and the internal truss frame structure can be designed to be sufficiently strong to meet any such load-bearing requirements. Preferably, the plate cover is designed only for fluid containment and for bearing local pressure loads. Separation of the two functions of a tank structure, i.e., the function of liquid containment fulfilled by the plate cover, and the overall tank stability and strength provided by the internal truss structure and the structural grillage of stringers and stiffeners permits use of thin metallic plates, e.g., up to 13 mm (0.52 in) for the plate cover. Although thicker plates may also be used, the ability to use thin plates is an advantage of this invention. This invention is especially advantageous when a large, e.g., about 160,000 meter3 (1.0 million barrel) substantially rectangular-shaped tank is built in accordance with this invention using one or more metallic plates that are about 6 to 13 mm (0.24 to 0.52 in) thick to construct the plate cover. In some applications, the plate cover is preferably about 10 mm (0.38 inches) thick.
Many different arrangements of beams, columns and braces can be devised to achieve the desired strength and stiffness of a truss frame structure as illustrated by the use of trusses on bridges and other civil structures. For a tank of the present invention, the truss frame structure construction in the longitudinal (length) and transverse (width) directions may be different. The trusses in the two different directions are designed to provide, at a minimum, the strength and stiffness required for the expected overall dynamic behavior when subjected to a specified seismic activity and other specified load bearing requirements. For example, there is generally a need to support the tank roof structure against internal vapor pressure loads and to support the entire tank structure against loads due to the unavoidable unevenness of the tank floor.
By using an internal truss frame structure to provide the primary support for the tank, the interior of the tank may be effectively contiguous throughout without any encumbrances provided by any bulkheads or the like. This permits the relatively long interior of the tank of this invention to avoid resonance conditions during sloshing under the substantially different dynamic loading caused by seismic activity as opposed to the loading that occurs due to the motion of a sea-going vessel.
In contrast to published designs of rectangular liquid storage tanks, which teach away from reinforcement and stiffening of tank walls in the vertical direction, the structural arrangement of the present invention permits use of structural elements such as stiffeners and stringers in both the horizontal and vertical directions to achieve good structural performance. Similarly, while published designs require installation of bulkheads and diaphragms to achieve required tank strength with such bulkheads and diaphragms causing large liquid sloshing waves during an earthquake and thus inducing large forces on the diaphragm structure and the tank walls, the open frame of the trusses in tanks according to this invention minimize dynamic loads due to liquid sloshing in earthquake prone sites.